The present invention relates to an improvement over prior methods and apparatus for surgically treating abnormal curvatures of the spine.
The normal spine possesses some degree of curvature in three different regions. The lumbar spine is normally lordotic (i.e., concave posteriorly), the thoracic spine kyphotic (i.e., convex posteriorly), and the cervical spine also lordotic. These curvatures are necessary for normal physiologic function, and correction is desirable when the spine has either too much or too little curvature in these regions as compared with the norm. A more common abnormality, however, is lateral deviation of the spine or scoliosis.
The first successful internal fixation method for surgically treating scoliosis involves the use of the Harrington instrumentation system. In this method, a rigid rod having hooks at each end is implanted adjacent the concave side of the scoliotic spine. The hooks engage in the facet joints of a vertebra above and under the laminae of a vertebra below the abnormally curved region. At the time of surgery, the spine is manually straightened to a desired extent. The distraction rod is then used to maintain the correction by exerting vertical forces at each end on the two aforementioned vertebra. The rod commonly has a ratcheted end over which the hooks are slidably mounted and locked in place. The effective length of the rod may thus be adjusted to an appropriate length for exerting the distractive force.
The Harrington distraction rod, because its corrective force is purely distractive, tends to correct curvature in both the frontal and sagittal planes. This means that unwanted loss of normal thoracic kyphosis or lumbar lordosis may inadvertently be produced. To compensate for this, a compression rod is sometimes placed on the convex side of the scoliotic spine. Another variation on the Harrington method which addresses the same problem is to contour the distraction rod in the sagittal plane in accordance with the kyphotic and lordotic curvatures of the normal spine. This may, however, reduce the ability to apply large corrective forces in the frontal plane due to column buckling.
The Harrington instrumentation system has been used successfully but exhibits some major problems. It requires a long post-operative period of external immobilization using a cast or brace. Also, because the distraction rod is fixed to the spine in only two places, failure at either of these two points means that the entire system fails. Failure at the bone-hook interface is usually secondary to mechanical failure of the bone due to excess distractive force.
Another method was thus developed utilizing the concept of segmented fixation. In this method, the spine is manually corrected to a desired degree as before. A rod is then fixed to the spine at multiple points by means of the sublaminar wires (i.e., wires running underneath the lamina of the vertebra and around the rod). The multiple fixation sites add to the stability of the system and make post-operative external immobilization frequently unnecessary. Segmental fixation also makes failure of the entire system much less probable. The possibility that loss of correction will occur post-operatively is also made less likely.
Segmental fixation may be used with a Harrington distraction rod or, as is more usually the case, with a pair of so-called Luque or L-rods. L-rods have a long segment which is aligned with the spine and a short segment perpendicular to the long segment. The short segments of the L-rods are inserted in notches or holes made in the spinous processes of vertebra above and below the deformed region of the spine. By placing the two L-rods on opposite sides of the spine and in opposite longitudinal orientation, the entire system is made less vulnerable to vertical migration.
Whether one rod or two is used in the segmental fixation method, the corrective forces are applied in a transverse direction via the sublaminar or spinous process wires rather than in a longitudinal direction as with a Harrington distraction rod. Since the corrective forces as applied transversely, the integrity of the system is not compromised when the rods are contoured to accommodate normal anatomic kyphosis and lordosis.
Another problem with both of the methods described above is their lack of effectiveness in producing rotatory correction in the transverse plane. The longitudinal forces of the Harrington distraction method, with or without an additional compression rod, do not contribute a corrective torque necessary for transverse plan derotation. The segmental fixation method could theoretically apply corrective forces in the transverse plane through the connecting sublaminar wires, but this is dependent on the sequence of wire tightening during implantation and is, as a practical matter, very difficult to achieve. This is unfortunate because scoliosis is generally a three-dimensional deformity requiring some correction in the transverse plane.
The shape-memory alloy, nitinol, has also been attempted as a Harrington rod without segmental fixation to correct scoliosis. This was unsuccessful because the corrective forces could not be transmitted effectively from the rod to the spine.
It is an object of the present invention to provide a method and instrumentation for the surgical treatment of scoliosis using segmental fixation which provides rotatory correction in the transverse plane.
It is a further object of the present invention to provide a method and instrumentation for applying corrective forces to the scoliotic spine while minimizing the forces which must be withstood by the fixation points, thereby lessening the possibility of metal bone interface failure.
It is a still further object of the present method to apply corrective forces to the scoliotic spine in a manner which minimizes the possibility of damage to the spinal cord.
It is a still further object of the present method to allow the easy technical insertion of an implant for correcting scoliosis by deforming the implant to match the shape of the patient's spine.